OP, it's a situation nobody wants to find themselves in and one nobody thinks will happen to them. And yet so many of us have been there.
I can only tell you what I'd do. I'd sit him down for a serious conversation. And I mean serious. I would make it clear this conversation could very well spell the end of our relationship, depending on what he said during it, and that I was fully prepared for that to be the outcome - you make no mistake about it, deadly serious.
I would tell him I'd noticed he'd been behaving strangely lately. If tell him not to even bother trying to convince me differently because I'm not an idiot, I know what I've seen and I know this is true. Don't even try and convince me I'm wrong. I'd tell him our relationship was hanging entirely on the explanation he was going to give me; not necessarily the content of the story, but whether or not it matched up to the facts I already knew (don't tell him what those are make out you may know far more than you actually do, from undisclosed sources) that would be it, no coming back from it, DONE. If there was anything I already knew that he didn't tell me, or that he lied about, or that he minimised, I wouldn't even be explaining to him how I knew he was lying, I would simply be gone. His only chance - ONLY chance would be to tell me everything and to trust that I would make a fair and reasonable decision of what to do next based on how honest I felt he'd been with me, not necessarily based on what he'd done. His ONLY chance.
And then, as he was telling me whatever story he eventually comes up with, I would be saying, "no, that's not it, that's not enough - you're walking on thin ice here mate, I am one second from leaving, you're going to have to do better than that" until he was literally a crying, quivering wreck, and certainly long after the first three times he said, "that's it, there's nothing else, I promise!" because the first few stories are never the truth, no matter how believable they sound. If he's gone to any lengths to hide it is bad enough that he'll be crying when he has to admit it.
But you have to mean it. You have to know lies mean the end. And then you just have to wait and see.
That's what I'd do. I know others would play it differently. Good luck.